Over the past few decades, semiconductor devices have become important elements in electronic industries due to their miniaturization, lightweight, multi-functional and other characteristics. Modern electronic industries may require semiconductor devices with more specialized functions, such as semiconductor-based transistors, solar cells, light-emitting diode (LED), silicon controlled rectifier, photodiodes, and digital and analog integrated circuits.
Photodiodes have been utilized in the areas including military, optical communication, information technology and energy. Photodiodes are operated by absorbing photons or charged particles to generate a flow of current in an external circuit, proportional to the incident power. An avalanche photodiode (APD) is a special type of photodiode, where incoming photons trigger an internal charge avalanche in APDs to generate an internal current gain effect (around 100) due to this avalanche effect. For producing optoelectronic devices such as photodiodes, a vapor phase diffusion process is often used to diffuse dopants into InP or InGaAsP wafers to create p-n junctions or local areas of high electron/hole concentration. More specifically, a zone of semiconductor material containing an excess of acceptor impurities resulting in a deficit of electrons or an excess of “holes” is said to exhibit P-type activity, while a zone containing an excess of donor impurities and yielding an excess of free electrons is said to exhibit N-type activity.
Dopants are generally implanted in two ways. In some processes, dopants may be implanted on the surface of a substrate and then heat treated to cause them to diffuse into the substrate. In other processes, dopants may be ionized into a plasma and then driven energetically into the substrate using an electric field. The substrate is then heat treated to normalize distribution of dopants and repair disruption to the crystal structure caused by ions barreling through at high speed. In both type of processes, the heat treatment anneals the substrate, encouraging dopant and ambient atoms located at interstitial positions in the crystal to move to lattice points. However, high energy implantation may drive ions to a deep portion of the substrate, but will generally not achieve conformal implantation and may result in over-implantation.
When the doping is carried out by thermal diffusion, a well-controlled diffusion process is necessary so that the physical and electrical properties of the diffusion layer have good controllability and reproducibility, and a large area of wafers can be diffused simultaneously. The well-control diffusion process may require good control of the source of doping materials to achieve conformal implantation and isolation from contaminants that can disrupt the diffusion process or adversely affect material quality.
U.S. Pat. No. 5,049,524 to Kuo et al. discloses a process for diffusing Cd into an InP substrate, which enables the use of a scaled-up sealed tube. The process uses simple apparatus and can diffuse a large number of wafers or a large surface area of wafers in a run, and thus meet the requirements for mass production and economic benefits, as shown in FIG. 1. However, in addition to heating a diffusion furnace to obtain a constant diffusion temperature zone, Kuo uses red P together with Cd3P2 to quickly increase the vapor pressure of phosphorous in the tube chamber to the saturated phosphorus pressure under the diffusion temperature to inhibit the evaporation of phosphorus molecules from wafer surface to further prevent thermal damages and ensure good controllability and reproducibility of diffusion. However, Kuo does not disclose any diffusion control means from the diffusion source to the wafers to achieve conformal deposition. Also, Kuo's diffusion control method may be limited to this specific manufacturing process.
U.S. Pat. No. 6,302,962 to Nam et al. discloses a diffusion system for manufacturing semiconductor devices has an air curtain formed across a furnace opening for preventing the loss of heat energy from inside the furnace, as shown in FIG. 2. The diffusion system maintains a uniform inner temperature inside a furnace, which improves the uniformity of the layers formed on a semiconductor substrate, increases the endurance of the furnace, and reduces the time for the temperature recovery, thereby improving the production yield of semiconductor devices. In other words, Nam uses the air curtain to compensate for the loss of the heated air flow and raise the inner temperature around the opening during the diffusion process to prevent the formation of non-uniform thickness layers formed on the semiconductor substrate. However, Nam focuses on improving thin film uniformity by controlling the temperature of the diffusion process, instead of controlling the diffusion path from the diffusion source to the wafers.
U.S. Pat. No. 7,964,435 to Ben-Michael et al. discloses a method for controlling dopant diffusion in a semiconductor manufacturing process. Ben-Michael controls certain parameters that are not used in the prior arts to control the diffusion process. For example, the size of a diffusion window (e.g. 310, 320) of a mask can be used to control diffusion depth. Also, the size of the diffusion window can be used in conjunction with other process variables, such as dopant concentration and ambient temperature, to provide a specific diffusion depth, as shown in FIG. 3. Likewise, Ben-Michael does not disclose or discuss the parameter of diffusion path from the diffusion source to the wafers and how to uniformly diffuse the material to the substrate. Therefore, there remains a need for a new and improved method and system to more effectively control the diffusion process.